Quantum Optoelectronics

Abstract:

Focused ion beam milled micropillars employing upper and lower distributed Bragg reflectors (DBRs) and incorporating InGaN quantum dots were analysed both microstructurally and optically. Comparison of the surface characteristics and the optical resonance of pillars milled employing two recipes, using comparatively higher and lower beam currents, were carried out through electron back scatter diffraction, atomic force microscopy and low temperature micro-photoluminescence. Low temperature micro-photoluminescence highlighted singly resolved InGaN quantum dot emission as well as modes with typical quality factors (Q) of ∼200-450 for typical 1-4μm diameter pillars, while one exceptional 4μm diameter pillar displayed optically-pumped lasing with a Q of ∼1100 at a threshold of ∼620 kWcm-2. The higher current recipe resulted in pillars with thicker surface amorphous layers, while the lower current recipe resulted in pillars with thinner surface amorphous layers but rougher surfaces. Micropillars milled through the recipe utilising higher beam currents were tentatively shown to possess lower Qs on average, correlating with the thickness of the surface amorphous layer. Finite difference frequency domain simulations in combination with analytical approximations of the various optical loss pathways suggested that surface scattering related optical loss was not significant compared to internal-based and surface absorption-based losses. The magnitude of the internal loss was observed to fluctuate significantly, which was thought to relate to the fluctuating micro-structure within the lower DBR and within the InGaN quantum dot layer. © 2012 American Institute of Physics.

Abstract:

Semiconductor nanopyramids (NPs) provide advantages in the development of novel functional optoelectronic devices due to their unique size-dependent properties. Here we demonstrate a new method for the fabrication of selectively self-assembled single-crystalline GaN NPs on the m-plane of periodically strained GaN/InGaN multiquantum disks embedded in the middle of GaN nanorods. The GaN NPs, which have ~100 nm diameters and heights, are observed by scanning electron microscopy and their crystalline structure is confirmed by high-resolution transmission electron microscopy. Experimental analysis directly reveals the strain distribution along the growth direction of the NPs. Cathodoluminescence measurements on a single NP show that its emission energy redshifts compared with that of bulk GaN, corroborating the results showing the formation of tensile strain in the NP. Observations of the uniform distribution and localization of these NPs show the possibility of further tuning their size and density by controlling periodically strained nanorod surfaces.

Abstract:

In this paper, we report on the design, fabrication and characterization of a broadband photonic crystal filter. Modeling with a genetic algorithm (GA) was used to investigate the effect of changing the number of periods and thickness ratios of a photonic crystal filter structure with two alternating materials. Theoretical optimized parameters were obtained as a function of wavelength for a photonic crystal filter with a very broad filter bandwidth as well as a very narrow transmission window. We used the determined optimum parameters at a wavelength of 1550 nm to fabricate the structure using e-beam lithography and inductively coupled plasma (ICP) etching. Experimental results show that the structure indeed has a very narrow transmission window and a low loss of just 4 dB. Hence, this structure can be regarded as a high precision filter for optical communication and photonic integrated chip technologies. © 2012 Elsevier B.V. All rights reserved.

Abstract:

Many intramolecular electron transfer (ET) reactions in biology are mediated by metal centers in proteins. This process is commonly described by a model of diffusive hopping according to the semiclassical theories of Marcus and Hopfield. However, recent studies have raised the possibility that nontrivial quantum mechanical effects play a functioning role in certain biomolecular processes. Here, we investigate the potential effects of quantum coherence in biological ET by extending the semiclassical model to allow for the possibility of quantum coherent phenomena using a quantum master equation based on the Holstein Hamiltonian. We test the model on the structurally defined chain of seven iron-sulfur clusters in nicotinamide adenine dinucleotide plus hydrogen:ubiquinone oxidoreductase (complex I), a crucial respiratory enzyme and one of the longest chains of metal centers in biology. Using experimental parameters where possible, we find that, in limited circumstances, a small quantum mechanical contribution can provide a marked increase in the ET rate above the semiclassical diffusive-hopping rate. Under typical biological conditions, our model reduces to well-known diffusive behavior.